Add a simplified class for fetching HTTP network content

This request adds a simplified, centralized class for fetching remote HTTP content and returning it as a QNetworkReply. It automatically handles creating a request using QgsNetworkAccessManager and following any redirects which may be in place.

While this pull request only changes composer html items to use the new class, this code is duplicated in different forms throughout QGIS. Centralizing it will make debugging and maintenance easier.